checks/debhelper.desc: Discuss debhelper's existing optimizations.

debhelper already includes optimizations to avoid running commands when
unnecessary, but people might not know that. Mention that in the help
for debhelper-excessive-overrides, and encourage people to improve those
optimizations in debhelper if needed.

Signed-off-by: Chris Lamb <lamby@debian.org>
1 job for master in 0 seconds (queued for 21 seconds)
Status Job ID Name Coverage
  External
passed jenkins

00:15:47